Table of contents

The dynamics in a system of impurity centres with a two-fold orbitally degenerate electronic state strongly coupled to the appropriate crystalline vibrations are investigated. The interaction between impurities occurs via the same phonons as participate in the electron-phonon coupling. A novel type of motion is shown to appear in the system owing to the interdependent reorientations of the centres between their energetically equivalent nuclear configurations. The influence of the interacting impurities on the elastic properties of the crystal is discussed.

The authors model the local electronic structure around a core-ionized site in a simple metal self-consistently using an embedding technique. They consider the local densities of states of Na, Mg and Al and calculate from first principles KLV Auger profiles. The success of this approach is assessed by comparison with experiment and with approximations which may be used to simplify the calculation. The agreement with experiment is on the whole good although some discrepancy arises for the case of KL_2,3V lineshapes.

The influence of electron correlation on the crystal-field splitting of Ho³⁺ in YVO₄, YAsO₄ and HoPO₄ is studied. Because the essential parts of the correlation effects cannot be attributed to spin correlation alone, the general correlated crystal field is used. Although this approach offers many parameters, the significant experimental results allow the determination of those parts of the electron correlation that are important for a description of crystal-field splittings.

For pt.I see ibid. vol.3, p.655-66 (1991). The influence of correlation effects on the crystal spectra of Ho³⁺ in LaCl₃ and Y(OH)₃ is investigated. It turns out that the variation of the crystal-field parameters from term to term is very similar to that found for the spectra of Ho³⁺ in YVO₄, YAsO₄ and HoPO₄. So the properties of the electron correlation in Ho³⁺ discovered for the zircon structure compounds are of more general validity and also apply for Ho³⁺ in LaCl₃ and Y(OH)₃.

The authors have used the complete neglect of differential overlap (CNDO) method to investigate the motion of the H atom within the Si lattice in the presence of a B impurity. Their results seem to indicate that the H atom moves in a low-energy channel in the vicinity of the B atom located at a substitutional site. However, the most stable configuration of this H-B system results when the H atom lodges itself near the bond-centred site of a Si-B bond (BM site). The B atom is displaced towards the plane of the other three neighbouring Si atoms and this displacement is calculated to be 0.28 AA, which agrees very well with the experimental value of 0.28+or-0.03 AA.

Annihilation spectra, resulting from the implantation of mono-energetic positrons in a perfect homogeneous solid, have been modelled allowing for the transport of both thermal and epithermal positrons. The resulting curves resemble a sum of two exponential components, although the intensities are not simply related to the positron back-diffusion probability. Because of the very short times involved, the effects of nonthermal positron transport are less significant than for steady-state diffusion models. At room temperature, internal reflection of thermal positrons at the surface has little effect on the solution to the diffusion equation for deeply implanted positrons.

A model of a DC- or AC-driven underdamped or overdamped commensurate CDW system with randomly distributed charged impurities, and a similar model of a driven damped or overdamped randomly inhomogeneous long Josephson junction are considered. The models are based on an underdamped or overdamped sine-Gordon equation including a driving term and a perturbation that describes a random lattice of local impurities. The lattice may be both sparse and dense, the latter being approximated by continuous random functions. A fundamental assumption is that initially the system contains frozen solitons (kinks) trapped by a disordered potential generated by the random lattice. With increase in the DC drive, the kinks are gradually released. The corresponding current-voltage characteristics (CVCs) are found. In the underdamped version of the model, the CVC proves to be hysteretic, while the overdamped version demonstrates a very pronounced threshold field. In the underdamped model, radiative dissipation is taken into account too. A dependence of AC conductivity on the AC frequency is also found for both models.

The authors report inelastic polarized neutron scattering measurements of the excitations in the S=1, quasi-one-dimensional antiferromagnet CsNiCl₃ with applied magnetic field. A field up to 5.9 T was applied perpendicular to the chain direction and the field dependence of spin excitation modes at wave vectors corresponding to the one-dimensional zone centre (i.e. ( xi , xi ,1)) was studied. Near the antiferromagnetic ordering point Q=(¹/₃,1/₃,1) two types of field dependence are observed. The behaviour of the higher energy mode suggests that it is the novel longitudinal mode recently proposed by Affleck (1989). The application of the field also reveals a near-degeneracy of the excitations observed at Q=(0,0,1) in zero field. This degeneracy is incompatible with the result of the classical spin wave theory based on the Heisenberg Hamiltonian with a single-site anisotropy.

The evolution with temperature of the magnetic structure of the C15 Laves-phase compound DyMn₂ has been studied using powder neutron diffraction and magnetization techniques. The Dy sublattice assumes a spin-canted ferromagnetic structure with 8.8 mu _B per Dy atom. Although all Mn sites within the unit cell are chemically equivalent, only one Mn atom in four is found to possess a magnetic moment (of 1.4 mu _B). These magnetic Mn atoms are located at sites with a strongly polarizing magnetic environment resulting from a near-neighbour configuration of ferromagnetically coupled Dy spins. A spin reorientation is observed at 36 K, and is accompanied by a small thermal expansion anomaly. The Curie temperature of DyMn₂ is found to be 45 K.

For pt.I see ibid. vol.1, p.179 (1989). An in situ ⁵⁴Mn nuclear orientation (NO) study of a quasi-1D antiferromagnet CsMnCl₃.4H₂O is presented and discussed. From a comparison between CsMnCl₃.2H₂O, and the 3D antiferromagnet MnCl₂.2H₂O, it is shown that there are significant differences in the NO behaviour. For example, in zero applied field, the ⁵⁴Mn nuclei in CsMnCl₃.2H₂O can be cooled to much lower temperatures than those in MnCl₂.4H₂O. However, in the presence of applied magnetic fields, a strong reduction in the NO signal is observed in CsMnCl₃.2H₂O, in marked contrast to MnCl₂.4H₂O in which rapid decrease in the nuclear spin-lattice relaxation time T₁ occurs. It is argued that many of the unexpected features observed in the quasi-1D antiferromagnet can be understood, at least qualitatively, in terms of both 'solitons' and a large zero-point motion of the Mn²⁺ spins. NO measurements taken in both the axial and equatorial directions reveal that there are about 12.5% spin flopped domains (solitons), in zero applied field. On the other hand, thermometric methods have been used to show that the hyperfine splitting of ⁵⁴Mn nuclei in CsMnCl₃.2H₂O is 356(15) MHz.

The temperature dependences of the quadrupole coupling (e²qQ/h) and relaxation time (T₁) for ²⁵Mg in metallic magnesium have been studied in the range 150 K to 400 K. The Korringa product T₁T=74(2) s K.

The authors introduce a new representation of the Bethe ansatz solutions for 1D Luttinger liquids which describes the spectral properties and asymptotic behaviour of the correlation functions of the 1D Hubbard model in terms of the renormalized interaction of charge and spin pseudoparticles. The study of the low-lying eigenstates is reduced to the familiar languages of band theory and of the Fermi liquid. The authors results provide a better understanding of the decoupling of charge and spin degrees of freedom in 1D interacting systems, generalize the concept of a Landau liquid and may be relevant to the physics of higher dimensional systems.

Photoemission spectra of the Al-Cu-Fe quasicrystal with the icosahedral phase were studied at room temperature. The line shape of the overall observed spectrum is like a large peak on a plateau cut-off at E_F. The large peak observed at about 4 eV below E_F seems to originate mainly from the 3d state of Cu atoms. The bump was observed at about 1 eV below E_F. The high resolution experiment showed an anomalous valley at E_F in the electronic density of states. This means that the dip at E_F in the DOS has a half-width 0.3-0.4 eV and that its depth is 70% of the value for a no-dip case.